Can I Tell His Girlfriend?

I have been dating a guy for two years. He has a serious girlfriend. I didn't know about the girlfriend until I was attached to him. I didn't leave even though I knew about the girlfriend. We tried to break up in January but in February he got in touch with me and talked me into seeing him again. This past Thursday I told him I needed a break. Our relationship has turned into a purely s*xual relationship. Now that we are having a break I want to tell the girlfriend. I have emails, IMs and specific information about her. I know that telling her is the wrong thing. I know that I'll never see him again if I do this. I know it could turn into a gigantic mess but I still want to do it. I don't want her to think that she "won" or she is better then me. I know that sounds crazy because she didn't even know about me. I don't even want to hurt the guy I want to hurt her. I know this makes me a lowlife and s so why do I want to do it so badly? Would anyone tell the girlfriend?